High Lockout
F.1 on Vaillant ecoMAX

Your boiler has detected a problem with the temperature sensor that monitors the water returning from your radiators, causing it to stop heating for safety.

Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed

Call an engineer soon

Book a Gas Safe engineer within 24 to 48 hours. Your boiler may be unsafe or could break down completely if left.

Technical description

Return–NTC: – NTC broken – NTC cable broken – Defective connection at NTC – Defective connection at electronics | Return–NTC cable defective/broken NTC faulty

What causes this fault?

This fault occurs when the piece of equipment responsible for measuring the temperature of the water returning to the boiler stops sending a clear signal. This is usually caused by the internal sensor failing due to age or a loose electrical connection in the wiring that links the sensor to the boiler's main control board.

Engineer required

This is a high severity fault. Stop using the boiler and call a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.

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Related fault codes

F.0View →F.10View →F.11View →

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